1. Field of the Invention
The present invention relates to a high differential pressure control valve and more specifically to a boiler feed water control valve for a variable pressure power operation boiler which operates under a wide range of load conditions.
2. Description of the Background Art
In general, a high differential pressure control valve for variable pressure operation employed in a boiler feed water control valve for a variable pressure boiler is arranged between a high pressure boiler feed pump discharge side and a boiler inlet for controlling a boiler feed water flow rate corresponding to a load on the boiler. Upon start-up of the boiler or during low load operation, the high differential pressure control valve is used in a severe high differential pressure and low flow rate condition for substantially low pressure at the boiler side. Conversely, during high load operation, it is necessary to assure a high flow rate under a low differential pressure condition since the pressure at the boiler side has risen close to the discharge pressure of the pump. When a fluid passes through a minimum throttle portion under low load operation, a static pressure decrease below a saturation pressure of the fluid due to an increasing flow velocity may cause cavitation which can result in substantial damage to the components of the valve.
In order to avoid damage due to cavitation, it has been proposed 1 to provide a porous-cage-type anti-cavitation valve having a special pore configuration as discussed in Japanese Examined Patent Publication No. 60-39911. 2 It has also been proposed to provide a variable orifice incorporating a throttle valve as discussed in Japanese Examined Patent Publication No. 2-11786. 3 Another proposed has been to provide a high differential pressure control valve as discussed in Japanese Examined Utility Model Publication No. 2-34533.
The flow control employing the anti-cavitation valve in 1 above is performed by moving a valve stem for intermittently opening and closing a plurality of special cage pores to provide a fixed flow resistance. Damage to the cage is also avoided by shifting the position of cavitation occurence away from the outlet of the pores of the cage. On the other hand, the variable orifice incorporating throttle valve in 2 and the high differential pressure control valve in 3 are high resistance valves having a multi-stage pressure reducing mechanism continuously associated with movement of the valve stem.
In the valve constructions in 1, 2 and 3 above, an unbalanced excessive thrust can be generated on the valve stem during a high differential pressure between inlet and outlet flow passage to require a substantial driving force for actuators connected to the valve stem. As a solution for this problem, the actual products employing the valve constructions in 1, 2 and 3 above, employ balancing by-pass holes formed in a valve plug for reducing the unbalanced thrust. By-pass holes however require balancing seals around the valve plug to avoid leakage of the fluid from the high pressure side to the low pressure side.
The valves discussed above have the following problems (1).about.(3).
(1) In the anti-cavitation valve of 1, since the flow rate control is performed by opening and closing the cage pores as fixed flow resistance, the flow characteristics becomes step-formed relative to the shifting magnitude of the valve stem to make it difficult to attain a high precision and continuous flow control. Also, since the cavitation is generated at the downstream side of the cage pore outlets, it can easily cause erosion on a seating surface of the valve plug.
(2) In the high resistance valves of 2 and 3 o above, since there is only high resistance flow passage from the valve inlet to the valve outlet, it is difficult to provide sufficient flow rate at a low differential pressure condition during high load operation of the boiler. Therefore, it is necessary to provide an additional large size low resistance control valve in parallel with the high resistance valve. This is disadvantageous from an economical viewpoint.
(3) In the valve constructions of 1, 2 and 3 above, there are further disadvantages associated with the necessity of providing balancing holes and balancing seals for reducing the thrust to act on the valve plug. (a) The balancing seal is subject to wear and damage due to foreign matter contained in the fluid resulting in leakage. The leakage of the fluid through the balancing seal may cause erosion to the valve components located in the vicinity of the balancing seal. (b) A sliding surface on which the balancing seal is mounted must be hardened and requires a mirror finish, which also increases manufacturing cost. (c) Substantially delicate work is required for disassembling and assembling the balancing seal thereby increasing maintenance costs.